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Trill code Male orange-eyed tree frogs trill to advertise their size not to prospective mates, but to other males, a new study has found.

"Frog calls are analogous to a human face in terms of the information they convey," says Dr Shinichi Nakagawa of New Zealand's University of Otago.

"They can convey 'sexy' information, such as whether someone is good looking or handsome, as well as the person's unique identity.

"Normally, frogs call to attract potential mates to their territory, however, the calls of orange-eyed tree frogs (Litoria chloris) are unique because they contain two distinctly different notes - moans and trills."

Previous research had suggested that the moan component attracts females, while the trills are thought to warn off other males.

"We wanted to determine exactly what information these frogs were transmitting in their calls."

Big frogs, small pond

The males of many species use costly signals to attract mates. These can range from growing ornate plumage, to elaborate nest construction, to calls that advertise their presence not only to the females they are seeking to impress but also to rivals and potential predators.

Orange-eyed tree frogs are found along the east coast of Australia north of Sydney. They call from beside flooded areas and shallow pools in rainforest streams during the breeding season, which runs from October to March.

Research team members from Griffith University recorded a range of calls during the 2011 breeding season, and analysed them to determine what information they contained.

Their study, led by Morgan Maclean and published in the Australian Journal of Zoology, found six different call features - trill frequency and duration, moan frequency, duration, number and number of pulses -combined to give each frog a unique call.

"We found that body size is correlated with trill frequency, that is, the bigger the frog, the deeper its trill," says Nakagawa. "However, its moan calls contained no size information that we could detect."

In many frog species size equates to quality, with larger males having grown faster or survived longer than smaller males. They usually have greater fertilization success and their offspring have better survival rates.

"Most frog calls contain information about the caller's size, as indicators of male quality that help females choose the best mate." This is known as intersexual indicator theory.

"These frogs are unusual because females don't choose mates based on body size, either large or small," says Dr Nakagawa. "If they did, that information would be contained in the moan calls."

Instead they seem to be using their calls only to keep in touch with the other frogs in their pond, helping to distinguish between friendly neighbours and potentially threatening strangers.